An important characteristic of human pain is that its intensity may change with time. In the experimental setting, a continuous or repeated noxious stimulation of the skin in humans may lead either to an augmented pain response (hyperalgesia) or to a reduction in pain (suppression and adaptation). We propose to measue the changes in pain sensitivity in humans that occur both during and following noxious heating or noxious mechanical stimulation of the skin. A new application of the method of successive intervals will enable the sensory magnitude of pain to be scaled against the dimension of time as well as physical intensity. Sequences of stimuli identical to those employed in the psychophysical experiments will be delivered to the receptive fields of high-threshold (nociceptive) afferents that innervate the monkey hand. The frequency and temporal pattern of discharge in these fibers during noxious stimulation will be compared with the time-varying estimates of pain evoked in humans by the same stimuli. In both experimental situations we will vary the time course and extent of preceding stimulations in order to trace the development of cutaneous hyperalgesia in humans (and possible sensitization of fiber response) and the recovery cycle of suppression of pain (and fatigue in fiber response). The objective of these experiments is to study the peripheral neural mechanims for encoding the intensity of cutaneous pain. If correlations are obtained between the proposed sensory and neurophysiological measurements, then justification is provided for the use of subhuman primates as a model for further studies of human pain.